The ink formulations of the present invention are employed in imaging processes which involve the application of liquid ink droplets in a pixel-by-pixel manner to an ink-receiving element. There are numerous schemes that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to the ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation.
The inks used in the various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant which is molecularly dispersed or solvated by the carrier medium. The carrier medium can be a liquid or a solid at room temperature. A commonly used carrier medium is water or a mixture of water and organic cosolvents. In dye-based inks, each individual dye molecule is surrounded by molecules of the carrier medium, i.e. no particles are observable under a microscope.
In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers, which serve to keep the pigment particles from agglomerating and/or settling out.
In addition to the colorant, other ingredients are also commonly added to ink jet inks. For example, cosolvents are added to help prevent the ink from drying out or crusting in the orifices of the printhead or to help the ink penetrate the receiving substrate, especially when the substrate is a highly sized paper. Examples of cosolvents which have been used in ink jet inks include glycols, including propylene glycol, polypropylene glycol, ethylene glycol, polyethylene glycol, diethylene glycol, tetraethylene glycol, and mixtures and derivatives thereof.
In ink jet inks which are fired through drop-on-demand thermal printheads, the amount of humectant should be minimized to prevent the ink viscosity from becoming too high, making the ink unable to be fired. High concentrations of humectant may also result in poor image quality and poor drying times. In ink jet inks, which are fired through certain types of drop-on-demand piezoelectric transducers, the viscosity of the inks tends to be higher than inks for thermal drop-on-demand inks. Typically, the viscosity of ink jet inks for use with shear mode piezoelectric printheads should be in the range of 4-20 centipoise. In the past, this has been achieved by adding high levels of humectants, typically in the range of 30-70 wt %. This can result in poor image quality, pigment and/or dye stability and degradation problems, and poor drying times.
In drop-on-demand ink jet printing it would be desirable to fire multiple droplets simultaneously at a given pixel location and have them merge, forming a single drop prior to reaching the receiver. As described herein, a discrete unit of ink issuing from a nozzle is defined as a "droplet." The merging of two or more droplets results in a "drop." After issuing from the nozzle, a droplet may break up into smaller particles and form an aerosol. These smaller particles are "subdroplets."
Image quality could be greatly improved, if one had the ability to fire any number of droplets, say between 0 and 16, at a given pixel location and have the droplets merge together to form a single drop prior to reaching the receiver. Currently, the simultaneous firing of more than one droplet from a given nozzle produces images that have poor image quality. This is due to the ink droplets not merging prior to reaching the receiver. It is possible to merge a small number of droplets, say 1-5 droplets, by increasing the low shear viscosity of the ink jet ink. This increase in viscosity is typically accomplished by increasing the amount of humectant in the ink. However, one would like to be able to merge a larger number of droplets, without the aforementioned tradeoffs of having large concentrations of humectants in the ink jet ink.
EP 0 787 778 A1 discloses the use of polyacrylamides and polyvinylpyrrolidones having a molecular weight of 10,000 and 30,000 respectively
"To effectively decrease the formation of aerosol, the viscoelastic polymer component is employed within the range of about 5 to 10 ppm. It has been determined experimentally that an ink-jet ink composition employing only 2 ppm of a polyacrylamide is not benefited by reduced aerosol formation. On the other hand, ink-jet compositions employing more than 10 ppm of a polyacrylamide exhibit pseudo-plastic effects (i.e., "rubberbanding") that are counterproductive to ink-jet printing. Moreover, precipitation might occur at concentratoins higher than about 10 ppm. Preferably, the viscoelastic polymer is employed at about 10 ppm." PA1 1. a pigment milling step in which the as-received pigment is deaggregated into its primary particle size, and PA1 2. a dilution step in which the pigment mill grind is converted into a useable ink. PA1 a) a liquid vehicle; PA1 b) 0.25 to 10.00 weight percent of a colorant; PA1 c) 1 to 500 ppm of a polymer dissolved in the vehicle; and PA1 d) 0 to 10 weight percent dispersant. PA1 a) 5 to 70 weight percent humectant; PA1 b) 0.0005 to 0.5 weight percent biocide; and PA1 c) 0.05 to 1.0 weight percent surfactant.
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While the conditions outlined in EP 0 787 778 A1 may help prevent a single droplet from breaking into sub-droplets (thereby forming an aerosol) (FIG. 1E), the molecular weight of the preferred and exemplified polymers is too low to facilitate the merging of multiple droplets to form drops, as is desired in the instant invention.
What is needed, then, is an ink jet ink formulation or method, other than the use of high concentrations of humectant, that will facilitate the merging of ink droplets before they reach the receiver.